The delivery of drugs to the eye presents many challenges. The ocular absorption of systemically administered pharmacologic agents is limited by the blood ocular barrier, namely the tight junctions of the retinal pigment epithelium and vascular endothelial cells. High systemic doses can penetrate this blood ocular barrier in relatively small amounts, but expose the patient to the risk of systemic toxicity. Topical delivery of drugs often results in limited ocular absorption due to the complex hydrophobic/hydrophilic properties of the cornea and sclera. Additionally, topical agents are mechanically removed by the blink mechanism such that only approximately 15% of a single drop is absorbed. Diffusion of topically administered drugs to the posterior chamber occurs, but often at sub-therapeutic levels. Intravitreal injection of drugs is an effective means of delivering a drug to the posterior segment in high concentrations. However, these repeated intraocular injections carry the risk of infection, hemorrhage and retinal detachment. Patients also find this procedure somewhat difficult to endure.
Local sustained delivery of therapeutics to the posterior chamber is critical in managing several chronic diseases of the eye. To address this need, several drug delivery devices have been developed for intraocular insertion into the vitreous region of the eye.
U.S. Pat. No. 4,300,557, for example, describes an intraocular implant in the form of a silicone capsule which can be filled with a drug to be delivered. The capsule is inserted in the vitreous region of the eye by making an incision in the eye, inserting the capsule and closing the incision. The capsule remains in place for a period of time and may be removed by making a second surgical incision into the eye and retrieving the device. The capsule has an attached tube which passes through the surface of the eye and extends outward from the eye useful for the subsequent injection of a drug. While in the vitreous, the device is not anchored and may move about freely.
U.S. Pat. No. 5,378,475 (often referred to as Vitrasert) describes a device which has been developed for insertion in the vitreous region of the eye, and is described in T. J. Smith et al., Sustained-Release Ganciclovir, Arch. Ophthalmol, 110, 255-258 (1992) and G. E. Sanborn, et al., Sustained-Release Ganciclovir Therapy for Treatment of Cytomegalovirus Retinitis. Use of an Intravitreal Device, Arch. Ophthalmol, 110, 188-195 (1992). This device consists of an inner core of pharmacologic agent surrounded by two coatings with different permeabilities. Drug diffuses through a small opening in one of these coatings achieving near-order release kinetics. It is implanted in the region of the pars plana through a 3.5-5.0 mm scleral incision. The implant must be removed and replaced every 6 months in the operating room as the drug becomes depleted. There is an approximately 25% complication rate from these procedures. The device is membrane diffusion drug delivery system that relies on EVA/PVA polymers to mediate release rate. Thus, many agents cannot be effectively delivered from such a system because their permeation rate through the rate controlling material of the system is too small to produce a useful effect. Other agents cannot be satisfactorily delivered by diffusional devices because of a particular chemical characteristic of the agent. This includes salts, because of their ionic character, and unstable polar compounds that cannot be formulated into a composition suitable for storage and delivery from such systems.
U.S. Pat. No. 5,098,443 describes a series of C-shaped rings that are inserted through incisions made in the eye wall or sutured around the globe of the eye. These rings may be formed from biodegradable polymers containing microparticles of drug. Alternatively, the implant may be in the form of a hollow flexible polymeric cocoon with the drug disposed therewithin for slow release by osmosis. No anchoring device is described.
U.S. Pat. No. 5,466,233 describes a tack for intraocular drug delivery. This device has an end that is positioned in the vitreous cavity while the head remains external to the eye and abuts the scleral surface. The drug is contained in the vitreous end of the device and could be contained within a biodegradable or nonbiodegradable scaffold. Alternatively, the device may have a hollow core filled with a drug that could diffuse through the wall of the tack into the eye. This core could be refillable. The head of the tack may further have a suture hole for anchoring the sclera.
While intraocular devices exist which allow delivery of therapeutic agents to the eye, a need still remains for a device which accomplishes controlled, sustained delivery to the posterior chamber, is implantable and removable without requiring long full thickness scleral incisions, does not cause undue patient irritation or discomfort, is stable within the vitreous region of the eye, is refillable and dose titratable, and is capable of delivering a wide range of small molecule, gene and protein therapeutics.